Stepper Motor FAQ Part 1

We receive a lot of questions from our customers on the topic of stepper motors, from completely general ones to more complex ones that require a deeper dive and teach us something new. Work with stepper motors, like any new field that we tackle and eventually try to conquer, starts with basic questions (what, how, why...) and then continues with more and more complex ones. In order to make it easy to find answers to the most common questions that we all face sooner or later, we have collected them in one place, in the article entitled The most frequently asked questions about stepper motors, part 1. The questions and answers are divided into two parts. The first part covers more general stepper motor FAQs, while the second part (Stepper Motor FAQ Part 2) builds on the knowledge and integration of the basic principles of stepper motor operation from Part 1.

1. What do the markings of the stepper motor Nema 17, Nema 23, Nema 34... mean?

It is not the name of a company that manufactures these types of stepper motors, as many would mistakenly think. The US National Electrical Manufacturers Association (NEMA) standardizes various dimensions, labeling and other aspects of stepper motors in the NEMA standard (NEMA ICS 16-2001). NEMA stepper motors are defined by the size of the faceplate. The NEMA 17 is a stepper motor with a 1.7 x 1.7 inch (43 mm x 43 mm) faceplate. There are other parameters to describe stepper motors, details can be found in the standard ICS 16-2001.

The most frequently asked questions about stepper motors have no standard — Nema 14 – Nema 34 sizes

2. When to use a stepper motor instead of a DC motor?

Stepper motors take steps based on individual pulses, and each step they take is approximately the same size. These pulses allow the motor to rotate at a precise angle (1.8° or less using microstepping (see question below)), which in turn maintains the precise position of the motor without the need for a feedback mechanism.
Where are stepper motors used? Stepper motors are used in industrial and commercial environments. They are the engine of choice for motion control applications that require high precision. They are popular for their low cost, high reliability and high torque at low speeds. They are simple, robust engines that can operate in almost any environment.

Positioning: 3D printers, CNC machines, XY positioning tables, conveyor belts on production lines, camera platforms, camera/camera lenses and some discs require precise positioning. Stepper motors are perfect for this, as they move in precise, repeatable steps.
Low speed torque. A stepper motor's maximum torque is always at low speed, making them an excellent choice for applications that require low speed without sacrificing accuracy. Classic DC motors usually do not offer the combination of low speed, high torque and high precision.
Controlled speed. The precisely controlled steps of stepper motors provide excellent control in automation and robotics processes

3. How do we choose a suitable stepper motor?

First of all, it is necessary to know what torque we need at a certain speed of rotation of the stepper motor. Once we have this information, we select a stepper motor based on two parameters:

of the holding torque of the stepper motor (it can be found in the specifications under "holding torque") and
speed-torque curves.

The value of the holding torque as a measure of the performance of the stepper motor must be taken with a margin, since the holding torque represents the torque that the motor produces at rest. As the stepper motor starts to rotate, its torque drops. Each stepper motor has its own characteristic speed-torque curve, which shows the drop in torque with increasing rotation speed.

Frequently asked questions about stepper motors, torque speed curve — Speed - torque graph of the stepper motor

The curve represents the torque at which the motor stops. I recommend doubling (100% safety factor) the required torque at a given speed, then choosing a motor/drive/power supply combination that can provide that torque (ie if we need 1.0 Nm of torque at a given speed, use a motor that produces 2 .0 Nm at this speed).

4. Why does motor size matter, why can't we just use a bigger stepper motor?

If the inertia of the motor rotor is the majority of the system inertia, all resonances become more pronounced (see the question about the relation between motor inertia and load in the next post). At the same time, a larger stepper motor needs more time to accelerate the rotor, which has more inertia, to the desired speed. So yes, smaller can be better.

5. How do we control the stepper motor?

A stepper motor system consists of a stepper motor, a stepper motor driver and a controller. Stepper motors rotate based on a train of pulses generated by the controller. In full-step mode, there are typically 200 steps (or 1.8 degrees per step) for each 360-degree revolution, and each pulse moves the motor one step. If we need more precision than 1.8 degrees and to avoid the resonance that is more common in this mode, the full step mode is not the right choice. This is why we usually choose half-step, quarter-step… modes that operate at 400, 800 or up to 50,000 steps per revolution. The controller is connected to the stepper motor driver, which then regulates the current through the stepper motor coils synchronously with the input pulses of the controller. The controller can be a "programmable logic controller" (PLC), a microcontroller or an even simpler pulse generator, such as an NE555 chip.

Frequently asked questions about stepper motors, stepper motor system — Stepper motor system

6. Is it safe to use a DC voltage higher than the rated voltage of the stepper motor?

Among the basic specifications of any stepper motor are rated current, winding resistance, torque, and rated voltage. Flow is important here. The torque produced is proportional to the current through the winding. The rated current is the current that produces the rated torque of the stepper motor. The rated voltage is easily calculated using Ohm's law from the rated current and winding resistance.

This is the voltage that, in a state of balance (if, for example, we connect the rated voltage directly to the coil and wait until the current stabilizes) will drive the rated current through the motor winding. The real situation in a stepper motor does not always represent a state of equilibrium, as the current changes rapidly. The inductance of the motor winding prevents rapid changes in the current, which thus takes time to rise to the value of the rated current. The longer this time, the lower the applied voltage. Therefore, in order to achieve high rotation speeds and at the same time sufficient torque, we use a voltage that is several times higher than the rated voltage of the stepper motor.

Example: Nema 23 step motor, 2.8 Nm of torque, nominal voltage 3.78 V, nominal current 4.2 A, phase resistance 0.9 Ω normally connect to a 36 V power supply. We used a voltage almost 10 times higher than the nominal voltage.

7. What are microstepping?

A typical stepper motor makes 200 steps in one 360-degree revolution (step size is 1.8 degrees). In full step mode, the stepper motor will thus make one full revolution for 200 input pulses from the controller. In this case, the rotation of the engine will be very "clunky" and quite loud due to resonance, especially at low speeds.

By regulating the current through the coils of the stepper motor, the driver allows a single 1.8 degree step to be divided into 2, 4, 8, 16, 32,... equal, small microsteps. This means that when selecting 1/32 microstepping, the smallest step the stepper motor can take is 0.056 degrees.

8. How does the power supply affect the maximum speed of the motor?

Each motor acts as a generator when rotating, sending voltage back to the driver. This is called “Electromotive force” or back EMF voltage. The magnitude of the EMF produced increases proportionally with the speed and inductance of the motor. This back EMF increases the impedance/effective resistance of the winding. Since Ohm's law still applies, this reduces the rms current flowing through the winding at a given voltage at the start of each step (I=U/R; current=voltage/resistance). In theory, when the back EMF equals the driver voltage, the motor stops. Based on this, we see that if we increase the voltage of the power supply, we can achieve higher speeds. However, a higher voltage may cause the motor to vibrate more at lower speed, and may also cause the overvoltage protection on the driver to trip or even damage the driver. Therefore, it is suggested to choose a supply voltage that is just high enough for the intended application.

9. How many stepper motors can be driven with one driver?

Most standard drivers are designed to drive a single stepper motor. In theory, we can connect more than one stepper motor to one driver, but this is not recommended at all. In this case, it is best to connect the windings of several motors in series, which will naturally limit the maximum possible speed. For higher speed and lower torque, it is necessary to connect the windings of several motors in parallel.

Frequently asked questions about stepper motors, stepper motor series connection — Series stepper motors. Image source: https://forum.v1engineering.com/t/stepper-motor-wiring/9812

10. What is the safe operating temperature of a stepper motor?

Most stepper motors have Class B insulation, which is rated at 130 °C. It is normal for stepper motor temperatures to reach 70°, 80° or even 90°C. While the engine is too hot to touch at these temperatures, the engine itself is undamaged. However, despite these facts, it is advisable to ensure that its temperature is as low as possible in order to extend the life of the stepper motor as much as possible. We can achieve this in several ways:

Reduction of quiescent current: Many motion control systems require maximum torque when the motor is accelerating and braking. When the engine is stationary or just holding its position, it needs much less torque. This is the right moment to reduce the motor current. Many stepper motor drivers allow the quiescent current to be reduced to 50 % of set current. A reduction in quiescent current can have a dramatic effect on motor temperature.

Workflow reduction: when choosing the right motor for our needs, we choose the motor with the maximum torque that is 100 % more than the torque we need. Therefore, there is usually no reason to select the maximum rated current of the stepper motor on the driver. It makes sense to choose such a current that provides the motor with a torque that is greater than needed for a certain reserve factor.

Using a closed-loop stepper motor system: a closed-loop stepper motor system uses a feedback loop to precisely control the current, speed, and position of the stepper motor. The current loop ensures that the current given to the motor by the driver is exactly what is needed for sufficient torque. When the motor is not producing torque (or producing less than maximum torque), the current is automatically reduced accordingly. This closed-loop control scheme significantly improves engine temperature.

Use of cooling fins and/or active cooling: very simple and affordable solutions that do not affect the amount of heat produced, but rather increase its removal.

11. Why is a stepper motor warm even at rest?

The stepper motor driver drives current through the windings of the stepper motor at all times, even when the stepper motor is at rest. This allows the stepper motor to produce torque without the intervention of a driver at rest when an external force tries to rotate its axis, thus maintaining a fixed position. Motor heating at idle can be reduced by setting the idle current to half on the driver (if it allows it).

12. Why does the stepper motor sometimes jump during power on or off?

The rotor of a stepper motor most often consists of two parts, each of which has 50 teeth. When current flows through the stator, the rotor moves so that its teeth align with the magnetized stator teeth. Since there are 50 natural positions of the rotor, it can move +/- 3.6 degrees in one of the directions.

Sources: